Method of inhibiting dissolution of calcium sulfate



Sept. 17, 1968 s. A. AXELRAD ETAL Re. 26,460

METHOD OF INHIBITING DISSOLUTION OF CALCIUM SULFATE Original Filed Aug.13, 1962 24 HOURS HOURS OLA I 1 l 1 I I I I l I 0 IO 3O 4O 5O 6O BO I00ADDITIVE CONCENTRATION (mq/ I I INCREASE IN CALCIUM CONCENTRATION (qpl)p WITHOUT ADDITIVE CALCIUM CONCENTRATION (9p I) 02 WITH ADDITIVE OII O 5l0 I5 20 25 FIG 2 TIME (nouns) United States Patent 26,460 METHOD OFINHIBITING DISSOLUTION OF CALCIUM SULFATE Bernard A. Axelrad, Raphael F.Matsou, and Freddie J.

Touro, by Calgon Corporation, Pittsburgh, Pa., a corporation ofPennsylvania, assignee, assignors, by mesue assignments, to CalgonCorporation, Pittsburgh, Pa., a corporation of Pennsylvania Original No.3,140,915, dated July 14, 1964, Ser. No. 216,591, Aug. 13, 1962.Application for reissue June 2, 1966, Ser. No. 569,766

8 Claims. (Cl. 23-89) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to a method for inhibiting the dissolution ofcalcium sulphate in aqueous solutions, and more particularly relates toa method for inhibiting the dissolution of calcium sulphate into a brineproduced by dissolving an alkali-metal halide, especially sodiumchloride, with water.

High purity brines are required for many processes and industrial uses.Sodium chloride brine, for instance, is used in the production ofmetallic sodium, chlorine, numerous sodium compounds, such as soda ash,caustic soda and sodium chloride, and is also used in the production ofsulphur by the Frasch process. One method of preparing brine is by thedissolution of underground salt masses with fresh or sea water which isinjected into the mass, the brine formed being withdrawn as aconcentrated solution.

All natural alkali-metal halides are relatively impure. Among the mosttroublesome contaminating materials is calcium sulphate which may bepresent in amounts up to 1% or more, that is, 10,000 parts per millionor more. Calcium sulphate is only slightly soluble in fresh water but ismore soluble in brine, its solubility increasing as the saltconcentration of the brine is increased. Calcium sulphate becomes lesssoluble in either fresh water or brine as the temperature of thesolution is raised. Thus, if a solution nearly saturated with calciumsulphate is heated, calcium sulphate precipitates gradually.Precipitation of the calcium sulphate during crystallization of saltfrom brine will cause contamination of the salt being crystallized.Moreover, the calcium sulphate tends to precipiate on the walls ofheaters and associated equipment as a glass-hard, continuous scale thatinterferes with heat transfer and reduces the open area of conduitsthrough the equipment.

The low solubility of calcium sulphate precludes economical removal ofthe scale from equipment surfaces. Not only is the calcium sulphate onlyslightly soluble in water, it is also impervious to acids and is veryslowly attacked by strong bases, even when hot. Mechanical methods aregenerally resorted to for its removal, but these must be so violent, dueto the extreme hardness of the scale, that the equipment being cleanedis frequently seriously damaged.

Solutions proposed to date to cure the problem have consisted inattempts to inhibit the precipitation of the calcium sulphate after ithas been dissolved in the solution. These methods have been found to beexpensive, time 'ice consuming and inefficient. Attempts have been madeto solve the problem by preventing the calcium sulphate from dissolvingin the brine in the first place. One method utilizes sodium carbonate orphosphate added to the aqueous solvent prior to bringing it into contactwith the contaminated halide to be dissolved. Another method utilizessodium polyphosphates in the same way. Both methods have thedisadvantage that if the water being used as the solvent containsappreciable calcium, the additive will be precipitated before itcontacts the calcium sulphate particles.

Accordingly, it is an object of this invention to provide a method forinhibiting the dissolution of calcium sulphate into an aqueous solventduring the dissolution of salt (i.e., sodium chloride or otheralkali-metal halide) into the aqueous solvent.

It is a special object of this invention to reduce the rate at whichcalcium sulphate is dissolved in aqueous solu' tions while dissolvingalkali-metal halides.

It is a further object of the present invention to provide a method ofpreparing a. brine from a solid salt, especially sodium chloride inwhich the dissolution of calcium sulphate into the resulting brine isinhibited.

Other objects will be apparent to those skilled in the art from readingthe following description taken in conjunction with the drawings, inwhich:

FIGURE 1 is a graph showing the effect of additive concentration on thesolution of calcium sulphate, and

FIGURE 2 is a graph which shows the influence of time and the use of oneof the additives of the invention on the dissolution rate of calciumsulphate.

The present invention results from the discovery that a class ofcompounds, unrelated to any of those known for the purpose before,reduces the rate of dissolution of calcium sulphate during the processof dissolving the alkalimetal halide with which the calcium sulphate isusually present. The members of this class of compounds vary from oneanother in cost, commercial availability, stability at elevatedtemperatures, and degree of calcium sulphate solution rate reduction,thus making it possible to choose from among the class, a compound whichis most economical and suitable for a particular purpose.

It has been found that the dissolution of calcium sulphate present as animpurity in natural alkali-metal halide formations can be inhibited byintroducing into the solvent small quantities of alkali-metal orammonium salts or hydrogen ion compounds which are made up of certainactive groups attached to at least one hydrocarbon group containingbetween 2 and 30 carbon atoms per active group. The active groups whichare useful -in the practice of this invention includes the sulfonates,sulfates, carboxylates, orthophosphates, phosphites, phosphonates, andphosphonites. The hydrocarbon group may be substituted or unsubstitutedand may be aliphatic, alicyclic, aromatic or heterocyclic.

It has been further found that for the inhibiting compound to be usefulin the practice of this invention, the product of its reaction with thecalcium present must be less soluble than calcium sulphate under theconditions of use.

The inhibiting compound used must be soluble to the extent of at least 2milligrams per liter of solution. The inhibiting compound may be presentat a concentration of 2 to 1000 milligrams or more per liter ofsolution,

and preferably is present in a concentration range of 5 to 100milligrams per liter. Amounts in excess of 1000 milligrams per literusually do not provide any useful purpose and are wasteful. The solutionin which the inhibiting compound is used should contain not less thanthree weight percent of the alkali-metal halide by the time that it iswithdrawn from contact with the undissolved calcium sulphate.

The inhibiting compound may be dissolved in the aqueous solvent beforethe aqueous solvent is brought into contact with the halide to bedissolved, or the calcium sulphate-contaminated alkali-metal halide maybe mixed with the compound in either its solid form or in concentrationsolution prior to bringing the water into contact with the alkali-metalhalide.

Inhibiting compounds found operable in the practice of this inventioninclude:

(1) Carboxylates:

(a) Sodium polyacrylate (b) Sodium carboxymethyl cellulose (c) Sodiumcaseinate ((1) Sodium salt of picolinic acid (e) Sodium salt ofZ-thiophene-carboxylic acid (f) Sodium salt of aminooctanoic acid (a)Sodium salt of benzenephosphonous acid Certain compounds are preferredbecause of their effectiveness, low cost, resistance to decomposition atD elevated temperatures of about 170 F., and their resistance toprecipitation by the ions initially present in sea water. The preferredcompounds are: dodecyl benzene sulfonic acid, sodium carboxymethylcellulose. sodium dodecyl benzene sulfonate, and sodium polyacrylate.

In the test represented by the data illustrated in FIG- URE 1, sodiumcarboxymethyl cellulose was added to synthetic sea Water in the amountsindicated. A saturated brine was produced by slowly agitating thesynthetic sea water in contact with excess sodium chloride containing1.3 weight percent of calcium as the sulphate. The amount of dissolvedcalcium sulphate present in the solvent was determined initially. Theincrease in calcium sulphate concentration at the end of five hours andat the end of twentyfour hours was determined and plotted on the graph.

FIGURE 1 shows that amounts as small as about 5 milligrams of theinhibiting compound per liter of solution, inhibited dissolution ofcalcium sulphate considerably. The decrease in the dissolution rate ofthe calcium sulphate is not, however, proportional to the concentrationof the inhibiting compound. Concentrations above about 15 milligrams ofinhibiting compound per liter. pro duced a less marked additionaleffect.

FIGURE 1 demonstrates that small concentrations of an inhibitingcompound greatly reduce the amount of Ill calcium sulphate whichdissolves during the saturation of an aqueous solution with sodiumchloride. FIGURE 1 also shows that greater concentrations of theinhibiting compound do not produce proportionally greater reductions incalcium sulphate dissolution.

The tests illustrated by the graph of FIGURE 2 were conducted byproducing saturated brine by slowly agitating distilled water in contactwith excess sodium chloride containing 1.3 weight percent of calcium asthe sulphate. The brine was prepared both with and without an additionof 20 milligrams of sodium dodecyl benzene sulfonate per liter ofsolution.

FIGURE 2 indicates that the added inhibiting compound reduces the rateat which the calcium sulphate dissolves. The solution, with theinhibiting compound present, continues to increase in calcium content asthe time of its contact with the calcium sulphate increases. However,the rate of increase of calcium content is much slower than that of asolution without the compound present.

The inhibiting compounds generally become more effective as the pH ofthe solvent is increased. Some of the compounds, for instance sodiumcaseinate and sodium polyacrylate, hydrolyze at low pH to poorly ionizedacids and become ineffective. With such compounds it is necessary toutilize a pH high enough to prevent hydrolysis.

Table I below illustrates that the compounds generally become moreeffective at higher pI-Is. The examples contained in Table I wereprepared by adding 750 milliliters of synthetic sea water. having a pHof 8, and containing 0.4 gram per liter of calcium as the sulphate. to450 grams of coarsely crushed sodium chloride salt core containing 1.3weight percent of calcium as the sulphate. Slow agitation of thesolution was started immediately upon addition of the solution to thesalt core. Samples of solution were taken at different time intervalsfor calcium determinations. Before starting the test, the inhibitingcompounds used for suppressing the rate of solution of calcium sulphatewere added to produce a concentration of 20 milligrams per liter in thesynthetic sea water, and hydrochloric acid or sodium hydroxide was usedto adjust the pH.

TABLE I.-(JALCIUM INUREASE IN URAMS PER LITER AFTER 24 HOURS SodiumSodium Sodium dodec p H Blank polyacrylate carboxymethyl yl benzenecellulose sulphunata D. 75 O. 75 O. 4 0. 6 0. 4 0. 3 0. 0. 4 0. 25 O. 25(l. 15 t]. 3 s r l). 05

In order to disclose more clearly the nature of the present invention,the following examples illustrating the invention are presented. Itshould be understood, however, that this is done solely by way ofexample and is intended neither to delineate the scope of the inventionnor limit the ambit of the appended claims.

EXAMPLE 1 To 450 grams (g.) of coarsely crushed sodium chloride saltcore containing 1.3 weight percent of calcium as the sulphate were added750 milliliters (mL) of distilled water. Slow agitation of the solutionwas started immediately upon adding the solution to the salt core.Samples were taken at different time intervals for calciumdetermination. At the end of twenty-four hours the calcium concentrationwas 0.85 gram per liter (g.p.l.).

The above experiment was repeated, except that 20 milligrams of sodiumdodecyl benzene sulfonate per liter were added to the distilled Waterprior to starting the test. At the end of twenty-four hours the calciumconcentration was 0.25 g.p.l. Thus, the concentration of calciumsulphate after twenty-four hours using the inhibiting compound was only29 percent as large as the calcium concentration where the inhibitingcompound was not used. 5 In carrying out the experiment, the liquid wasstirred at a slow rate so that only the smallest particles were solutionwas started immediately upon adding the solution to the salt core.Samples were taken for calcium determinations. The inhibiting compoundsused for suppressing the rate of solution of calcium sulphate were addedto produce a milligram concentration per liter of total solution. Thecalcium increase in grame per liter after twenty-four hours contact timeis shown in Table III.

TABLE III Calcium Ex. Class Representative compound incroalse,

Blank (no compound added) 0. 4 ll Sulphonate. Sodium dodocyl benzenesulphonate.. 0. 15 12 Sulphate... sodilurln T-etllyl, ZmethyIundecylt-0.15

su p late.

13 Carhoxylate Sodium cnscinate 0.15 l4- Orthophosphate-.. Sodium saltof dibutyl phosphate. 0.25 Phosphite Sodium salt of dipheny phosphite.-0.3 16 Phosphonate sodijulm salt of benzene pliosphonic 0. 3

17 Phosphonite Sodium salt of benzene phosphonous 0. 3

18 Sulphonate-carboxylate Disodlurn N-octadecylsulphosucclna- 0. 2

ma e.

19 Unsubstituted hydrocarbon. Sodium polyacrylatc 0.25

Substituted hydrocarbon. Sodium carboxymethyl cellulose 0.25 21 Acidform Dodecyl benzene sulphonic acid 0. i5 22 Ammonium form Ammoniumpolyacrylate 0.25

barely lifted and moved in the solution. It was found that the rate ofsolution of calcium compounds was affected by conditions of agitation.It was also found that regardless of the speed of agitation the additionof the inhibiting compounds always showed a marked reduction in the rateof dissolution of the calcium sulphate into the solution.

EXAMPLES 2-10 Table III, in Examples 11 through 18, shows the activegroups which are operative in the practice of this invention. Examples19 and 20 show that the hydrocarbon group to which the active group isattached may be either unsubstituted (sodium polyacrylate) orsubstituted (sodium carboxymethyl cellulose).

Table III also shows that various forms of the compounds may be used.Examples 11 through 20 demonstrate the effectiveness of the alkali-metalform. Example 21 demonstrates the effectiveness of the acid form, andExample 22 demonstrates the effectiveness of the ammonium form of thecompound.

Based on the above findings, it was determined that a brine havingreduced calcium sulphate impurity may be produced by the dissolution ofunderground salt deposits by a procedure similar to the following.First, an inhibiting compound is added to the aqueous solvent, which maybe fresh water or sea water, either natural or syn- TABLE II thetic.Second, the aqueous solvent is injected into the Calcium salt bearingformation in the usual manner well known in Example Inhibiting CompoundUsed grams ne r the art. Third, the brine is removed from the saltbearing formation in the usual manner. The resulting brine is Blank(nocompound added) 0.8 lower in calcium sulphate content than brineprepared oifii c fiii dtifflflfff fiTil;..::::::::: 8:3? Without theaddition of the inhibiting p n to the 4.-. Sodlumisopropylnapthalenesulphonate 0.25 aq-uegug solv t, $33 2'methyl'undecyl'4'sul Thefollowing example illustrates the preparation 6m DisodiumNmiadecylsulphosucvinamflw-- 35 [prepartion] of a brine of reducedcalcium sulphate con- 1 b s 1- 0. as 7 5,533.? ma oxydi amen di u tentfrom an underground salt mine. 8 gogiumalklylarylpolyethersulphonate8.2g 9. o ium p0 yacry ate 10 Sodium carboxymethyl cellulose 0.65EXAMPLE 23 Table II shows the relative effectiveness of the variousadditives in inhibiting the dissolution of calcium sulphate into anaqueous solvent containing an alkali-metal halide.

EXAMPLES ll-l2 Using ordinary sea water, containing initially on anaverage of about 1400 mg. of calcium sulphate per liter, there was addedapproximately 10 mg. of the sodium salt of carboxymethyl celluloseadditive per liter of sea water. The sea water containing the additivewas pumped into the large cavity of an underground salt mine. Previousto the test, brines produced by pumping the same sea water, without thesodium salt of carboxymethyl cellulose additive, produced a brinecontaining 3700 mg. of calcium sulphate per liter of brine. When theoperation was conducted using the same sea water containing the 10 mg.of sodium salt of carboxymethyl cellulose per liter, the brine producedwas found to contain progressively lesser amounts of calcium sulphateduring the brine-producing operation as indicated by Table IV below.

TABLE I" Number of days of operation: Ammmt f mlt'lum S111 pirate in theresulting brine, mg. CaSO. per

This test, which was conducted under actual brine-forming conditions,shows the marked reduction in the amount of calcium sulphate which isdissolved in forming the brine. The reason for the progressive effectwas that the cavity in the underground mine was so large that it tooktime to provide an adequate concentration of sodium salt ofcarboxymethyl cellulose in the mine since it was diluted by wateralready in the mine. The progressive reduction in content of calciumsulphate in the brine lowered the Calcium sulphate content to a levelwhich made the resulting brine usable for sulphur mining by the Eraschprocess.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions of excluding any equivalents described orportions thereof, but it is recognized that various modifications arepossible within the scope of the invention claimed.

What is claimed is:

1. A method for inhibiting the dissolution of calcium sulphate into anaqueous slovent comprising dispersing into the aqueous solvent aninhibiting compound having at least one active group selected from theclass consisting of [sulphonate, sulphate, carboxylate, phosphate,]phosphite, phosphonate and phosphonite radicals, said active group beingattached to at least one hydrocarbon group containing from about 2 to 30carbon atoms per active group, said compound combining with calcium toform a product that is less soluble than calcium sulphate.

2. A method according to claim 1 wherein from 2 to 1000 milligrams ofsaid inhibiting compound are dissolved in each liter of solvent.

3. A method according to claim 1 wherein from 5 to 100 milligrams ofsaid inhibiting compound are dissolved in each liter of solvent.

4. A method according to claim 1 wherein the calcium sulphate is presentwith alkali-metal halides and the solution produced contains at least 3weight percent of the alkali-metal halide.

[5. A method according to claim 1 wherein said compound is dodecylbenzene sulphonic acid] [6. A method according to claim 1 wherein saidcompound is sodium carboxymethyl cellulose] [7. A method according toclaim 1 wherein said compound is sodium dodecyl benzene sulphonate.]

[8. A method according to claim 1 wherein said compound is sodiumpolyacrylate] [9. A method according to claim 1 wherein said compound issodium caseinate] [10. A method according to claim 1 wherein saidcompound is sodium isopropyl naphthalene sulphonate] [11. A methodaccording to claim 1 wherein said compound is sodium 7-ethyl, 2methyl-undecyl-4- sulphate] [12. A method according to claim 1 whereinsaid compound is disodium N-octadecyl sulphosuccinamate] [13. A methodaccording to claim 1 wherein said compound is sodium dodecylatedoxydibenzene disulphonateJ [14. A method according to claim 1 whereinsaid compound is a sodium alkyl aryl polyether sulphonate.]

[15.] 5. A method of preparing brine from an aqueous solvent and a solidalkali-metal halide containing calcium sulphate as an impuritycomprising adding to the aqueous solvent no later than simultaneous withthe contact of the solvent with the halide a compound having at leastone active group selected from the class consisting of [sulphonate,sulphate, carboxylate, phosphate,] phosphite, phosphonate andphosphonite radicals, said active group being attached to a hydrocarbongroup containing between about 2 to 30 carbon atoms per active group,said compound combining with calcium to form a prod net that is lesssoluble than calcium sulphate.

[16.] 6. A method of preparing brine from an aqueous solvent and a solidalkali-metal halide containing calcium sulphate as an impuritycomprising adding to the solid alkali-metal halide no later thansimultaneous with the contact of the solvent with the halide a compoundhaving at least one active group selected from the class consisting oflsulphonate, sulphate, carboxylate, phosphate,] phosphite, phosphonateand phosphonite radicals, said active group being attached to ahydrocarbon group containing between about 2 to 30 carbon atoms peractive group, said compound combining with calcium to form a productthat is less soluble than calcium sulphate.

[17.] 7. A method according to claim 1 wherein said inhibiting compoundis soluble to the extent of at least 2 milligrams per liter of solution.

[18.] 8. A method according to claim 1 wherein said inhibiting compoundis in the form of a salt selected from the class consisting of thealkali-metal and ammonium salts.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

NORMAN YUDKOFF, Primary Examiner.

S. I. EMERY, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Reissue No. 26,460September 17, 1968 Bernard A. Axelrad et a1.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 5, line 65, "EXAMPLES 11-12" should read EXAMPLES 11-22 Column 6,line 6, "grame" should read grams Column 7, line 23, "Erasch" shouldread Frasch line 34, "slovent" should read solvent Signed and sealedthis 17th day of February 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. Attesting Officer Commissioner of Patents

